The present invention relates to a control unit of an automatic transmission for a vehicle with a continuous transmission mechanism, and more particularly, to a transmission control unit of the continuous transmission mechanism.
When a vehicle is on a road low friction surface (hereinafter "low-.mu."), such as a road on which snow is accumulated, a frozen road, or a gravel road, it often occurs that the driving force of a driving wheel of the vehicle exceeds the frictional force of the tire gripping the road surface and that the vehicle skids.
Conventionally various kinds of means have been proposed to prevent a vehicle which is running from skidding. Particularly, in an automatic transmission in which a continuous transmission is used as a transmission portion, an anti-skid unit which is disclosed in Japanese Laid-Open No. SHO 60-192155 has been proposed. In this anti-skid unit, a method is adopted that skidding of a driving wheel is detected and the torque ratio of the continuous transmission is controlled and a shift is made so that the driving force may be lowered until the driving wheel ceases to skid.
However, according to the anti-skid unit as above referred to, as the shift is made after skid of a driving wheel is detected when the vehicle starts to move or when the vehicle is moving on a low-.mu. road, there is a problem that there is a predetermined length of a time-lag until the shift ratio reaches a required value and skidding during that period can not be prevented.
Further, when the vehicle starts to move, as the transmission is in an under-drive state and a large torque acts on the driving wheels, skidding is likely to occur all the more compared with a case when the vehicle moves at a constant speed. However, with the shift after detecting a skid in the above manner, a smooth starting of movement can not be expected.